The invention disclosed herein generally relates to bond coatings and thermal barrier coatings applied to metals. The metals are frequently portions of components used in turbine engines. The invention also relates to processes for depositing such coatings.
Components formed of specialty materials like superalloys are used in various industrial applications, under a diverse set of operating conditions. In many cases, the components are provided with coatings which impart several characteristics, such as corrosion resistance, heat resistance, oxidation resistance, and wear resistance. As an example, the various components of turbine engines, which typically can withstand in-service temperatures in the range of about 1100xc2x0 C.-1150xc2x0 C., are often coated with thermal barrier coatings (TBC""s), to effectively increase the temperature at which they can operate.
Most TBC""s are ceramic-based, e.g., based on a material like zirconia (zirconium oxide), which is usually chemically stabilized with another material such as yttria. For a jet engine, the coatings are applied to various superalloy surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles. Usually, the TBC ceramics are applied to an intervening bond coating (sometimes referred to as a xe2x80x9cbond layerxe2x80x9d or xe2x80x9cbond coatxe2x80x9d) which has been applied directly to the surface of the metal part. The bond coat is often critical for improving the adhesion between the metal substrate and the TBC.
The effectiveness of a TBC is often measured by the number of thermal cycles it can withstand before it delaminates from the substrate which it is protecting. In general, coating effectiveness decreases as the exposure temperature is increased. The failure of a TBC is often attributed to weaknesses or defects related in some way to the bond coating, e.g., the microstructure of the bond coating. TBC failure can also result from deficiencies at the bond coating-substrate interface or the bond coating-TBC interface.
The microstructure of the bond coating is often determined by its method of deposition. The deposition technique is in turn determined in part by the requirements for the overlying protective coating. For example, many TBC""s usually require a very rough bond coat surface (e.g., a root mean square roughness (Ra) value of greater than about 200 micro-inches), for effective adhesion to the substrate. An air plasma spray (APS) technique is often used to provide such a surface.
There continues to be a need in the art for bond coatings which provide very good adhesion between the substrate and a subsequently-applied TBC, e.g., bond coatings with a relatively rough surface. Furthermore, new processes for applying and curing such coatings in regions of a substrate which are somewhat inaccessible are also of great interest. (Conventional thermal spray equipment is sometimes too large and cumbersome for such regions). Moreover, the entire TBC systemxe2x80x94bond coating with the TBC itselfxe2x80x94should exhibit good integrity during exposure to high temperatures and frequent thermal cycles. Such a system should be effective in protecting components used in high performance applications, e.g., superalloy parts exposed to high temperatures and frequent thermal cycles.
One embodiment of the present invention is directed to a method for applying a bond coat on a metal-based substrate, comprising the following steps:
a) applying a slurry which comprises braze material to the substrate, wherein the slurry also contains a volatile component;
b) applying bond coat material to the substrate;
c) drying the slurry and bond coat material under conditions sufficient to remove at least a portion of the volatile component; and
d) fusing the braze material and bond coat material to the substrate.
The braze material is usually based on nickel, cobalt, or iron. The bond coat material is often an xe2x80x9cMCrAlXxe2x80x9d material or a metal carbide, as discussed below.
There are a variety of methods for applying the bond coat according to this invention. One method calls for combining the bond coat material and the braze material with a solvent and one or more additives, as described below. The combined slurry mixture can then be deposited on the substrate by various techniques, such as flow-coating, brushing, or spraying. As an alternative, the slurry applied in step (a) includes the braze material but not the bond coat material, and is substantially dried to form a green layer. An adhesive can be applied to the green layer, and the bond coat particles can then be applied to the adhesive, prior to the fusing step. As another alternative, two separate slurries can be employedxe2x80x94one containing the braze material, and the other containing the bond coat material. Each slurry can contain the additives described below. In this embodiment, the braze slurry is usually applied first, followed by the application of the bond coat slurry. The slurries can then be dried and fused to the substrate. An overcoat can optionally be applied over the bond coat. The overcoat is usually a conventional thermal barrier coating, e.g., one based on zirconium. Alternatively, the overcoat can be of another type, such as a metal carbide-based wear coating.
A method for replacing a bond coat applied over a metal-based substrate is also described below. The following steps are usually included in this method:
(i) removing the existing bond coat from a selected area on the substrate;
(ii) applying a slurry which comprises braze material to the selected area, wherein the slurry also contains a volatile component;
(iii) applying additional bond coat material to the selected area; and
(iv) fusing the braze material and bond coat material to the selected area.
This technique can be part of the overall process for repairing a worn or damaged TBC system.
Another embodiment of this invention is directed to a unique slurry composition, containing a braze material and a bond coat material, along with other conventional slurry ingredients, such as a solvent. As discussed elsewhere, the braze material is usually nickel, cobalt, iron, a precious metal, or some mixture containing one of those components. The bond coat material is usually of the MCrAlX-type (discussed below), or can be a metal carbide or other type of material. The slurry composition is very useful in the formation of a TBC system.
An article constitutes another embodiment of this invention. It comprises:
(a) a metal-based substrate, and
(b) a volatile-containing slurry on the substrate, comprising braze material and bond coat material (e.g. roughness-producing bond coat particles).
The substrate is often a superalloy, and the braze material and bond coat materials are as described below. When the volatile component in the slurry has been substantially removed, a green coating remains, which is fused to the substrate, e.g., by brazing. As fused, the braze material forms a continuous matrix phase in which the bond coat particles are embedded.
Other features and advantages of the present invention will be more apparent from the following detailed description of the invention.